This invention relates to compositions of soda-lime type glass with low luminous transmittance; typical transmissions for a 4 mm thick sheet do not exceed 20%.
The expression “soda-lime glass” is used here in the broad sense and relates to any glass containing the following constituents (percentages by weight):
SiO266 to 75%Na2O10 to 20%CaO5 to 15%MgO0 to 5%Al2O30 to 5%K2O0 to 5%
This type of glass is very widely used in glazing for buildings or motor vehicles, for example. It is usually manufactured in the form of a ribbon by a float process. The ribbon can then be cut into sheets which can then be curved or undergo a treatment to strengthen their mechanical properties, such as thermal tempering.
It is generally necessary to refer the optical properties of a sheet of glass to a standard illuminant. In this description, two standard illuminants are used: illuminant C and illuminant A as defined by the International Commission on Illumination (CIE). Illuminant C represents average daylight with a colour temperature of 6 700 K. This illuminant is especially useful for evaluating the optical properties of glazing for buildings. Illuminant A represents the radiation from a Planck radiator at a temperature of around 2 856 K. This illuminant simulates the light emitted by automobile headlamps and is generally used to evaluate the optical properties of automotive glazing. The International Commission on Illumination also published a document entitled “Colorimetry, Official Recommendations of the CIE” (May 1970) describing a theory according to which the colorimetric coordinates for light of each wavelength in the visible spectrum are defined so that they can be represented on a diagram with orthogonal x and y axes, known as the CIE 1931 tri-chromatic diagram. This tri-chromatic diagram shows the region representing the light of each wavelength (expressed in nanometres) of the visible spectrum). This region is known as the “spectrum locus” and light that has coordinates on this spectrum locus is said to have 100% excitation purity for the appropriate wavelength. The spectrum locus is closed by a line known as the line of purples which joins the points of the spectrum locus whose coordinates correspond to the wavelengths 380 nm (violet) and 780 nm (red). The area between the spectrum locus and the purples line is the area available for the tri-chromatic coordinates of any visible light. The coordinates of the light emitted by illuminant C, for example, correspond to x=0.3101 and y=0.3162. This point C is considered to represent white light and therefore has an excitation purity of zero for any wavelength. Lines can be drawn from point C towards the spectrum locus at any desired wavelength and any point is located on these lines can be defined not only by its x and y coordinates, but also as a function of the wavelength corresponding to the line on which it is found and its distance from point C in relation to the total length of the wavelength line. Therefore, the hue of the light transmitted by a sheet of coloured glass can be described by its dominant wavelength and its excitation purity expressed as a percentage.
The CIE coordinates of light transmitted by a coloured sheet of glass will depend not only on the composition of the glass but also on its thickness. In this description, and in the claims, all values of excitation purity P and dominant wavelength λD of the transmitted light are calculated from the specific internal spectral transmittance (TSIλ) of a 5 mm thick sheet of glass. The specific internal spectral transmittance of a sheet of glass is governed only by the absorption of the glass and can be expressed by the Beer-Lambert law:
TSIλ=e−E.Aλ where Aλ is the absorption coefficient of the glass (in cm−1) at the wavelength considered and E the thickness of the glass (in cm). As a first approximation, TSIλ can also be represented by the formula(I3+R2)/(I1−R1)where I1 is the intensity of the incident visible light at the first face of the glass sheet, R1 is the intensity of the visible light reflected by that face, I3 is the intensity of the visible light transmitted from the second face of the glass sheet and R2 is the intensity of the visible light reflected back inside the sheet by the second face.
In the following description and the claims, we also use:    total luminous transmittance for illuminant A (TLA), measured for a thickness of 4 mm (TLA4). This total transmittance is the result of integrating between the wavelengths of 380 and 780 nm the expression: Σ Tλ.Eλ.Sλ/Σ Eλ.Sλ in which Tλ is the transmittance at wavelength λ, Eλ is the spectral distribution of illuminant A and Sλ is the sensitivity of the normal human eye according to the wavelength λ.    the total energetic transmittance (TE), measured for a thickness of 4 mm (TE4). This total transmittance is the result of integrating between the wavelengths of 300 and 2500 nm the expression: Σ Tλ.Eλ/Σ Eλ in which Eλ is the spectral energy distribution of the sun at 30° above the horizon.    the selectivity (SE), measured as the ratio of the total luminous transmittance for illuminant A to the total energetic transmittance (TLA/TE).
There is demand for glazing with high light absorption in both buildings and automotive applications. In addition to low luminous transmittance, very low energetic transmittance is also usually required.
The choice of compositions also involves colour transmittance and reflection characteristics. Demand for highly absorbent glass mainly relates to sheets offering good neutrality. This is characterised by the degree of colour purity. A neutral coloration corresponds to a purity well below 10.
Making glass satisfying a set of conditions of the type indicated above involves a particularly delicate choice of colouring materials, especially as, in addition to the performance of glass made with these compositions, there are also conditions affecting the choice of materials that can be used, taking account in particular of the requirements related to the manufacturing techniques. For instance, colouring materials designed to absorb infrared radiation tend to prevent the attainment of uniform temperatures in the baths of the melting furnaces, which are heated from above by radiation.